ARM: dts: imx50: Add PHY node for usbotg and adjust clocks

Even though the ChipIdea USB controller binding[1] doesn't specify the
properties that reference a PHY as required, the Linux driver
requires[2] such a reference.

The clock situation is like on i.MX53: The USB controller is clocked
from IMX5_CLK_USBOH3_GATE and the PHY from IMX5_CLK_USB_PHY1_GATE.

[1]: Documentation/devicetree/bindings/usb/ci-hdrc-usb2.txt
[2]: Search for EINVAL in drivers/usb/chipidea/ci_hdrc_imx.c

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>